Ion-induced oil–water wettability alteration of rock surfaces. Part III: Ion-bridging interactions between oil and solid

We reveal the effects of ions on the oil–water wettability of rock surfaces, which originally adsorb polar molecules via ion-bridging interactions. The ion-induced oil-water wettability alteration is instead revealed by using molecular dynamics simulations, owing to the difficult in the realization of ion-bridging interactions in a hydrated water film in experiments. The results show that the spontaneous displacement of Ca²⁺ ions in water film by Na⁺ ions in bulk water phase can weaken the ion-bridging effect and accordingly the rock surface becomes more water-wet. A maximum decrease of approximately 20° in the water contact angle is observed in a period of 30 ns at 0.5 mol/L of Na⁺ ions. The displacement is guaranteed because smaller Na⁺ ions can easily embed into silica structure and subsequently weaken the electronegativity of rock surfaces. This study gives a molecular insight of the mechanism of wettability alteration on rock surfaces under ion-bridging interactions.